Fundamental to any common accounting package is the double-entry bookkeeping system.
Currently, CiviCRM does not, however, have a double-entry recording system. Contributions are stored in civicrm_contribution_* tables.
Preferably, when someone makes a Contribution the transaction would be stored in an income account and also in an Accounts Receivable or Cash/Bank account.
This way, you can quickly determine what your income is for a particular period and how much is still owed to you. Pledges could therefore be a form of unpaid Contribution that would go into the Accounts Receivable account.